Humans and other animals have the remarkable ability to negotiate an unknown and changing environment, not only without falling, but also with a level of efficiency currently unmatched in existing robotic systems. Literature suggests that a large portion of this agility may be due to the natural behavior of the animal's body in addition to neurological feedback control. Morphology and materials of limbs (their mass, elasticity of tendons, lever arms between joints, etc.) can enable efficient and immediate feedback control and stabilization at the hardware level. Using this hardware-in-the-loop control premise, natural looking walking and running gaits can emerge from the “natural” (“free” or “passive”) dynamics of the mechanism.
Passive dynamics exist for any physical system, whether premeditated or not, and whether favorable or detrimental to the task at hand. For highly dynamic behaviors, with large accelerations, impacts, and/or energy transfers, passive dynamics can significantly affect the performance. Very high bandwidth, high power actuators, such as those in hydraulic machines, are sometimes capable of overcoming unfavorable passive dynamics and exhibiting highly dynamic behavior, but they do so at the cost of extremely high power requirements. Designing systems with appropriate passive dynamics can greatly simplify the active control system, permit low-bandwidth actuation, and minimize energy costs. However, utilizing passive dynamics also constrains the behavior of the system to only those dynamics embodied by hardware, and cannot change without morphological changes. As such, machines that focus exclusively or extensively on passive dynamics rather than computer control will have only a single behavior, such as a single gait at a single speed on flat ground, and may be very susceptible to disturbances such as small bumps in the ground.
In one existing approach to legged locomotion, robustness and agility may be obtained by minimizing the effect of passive dynamics as much as possible, and utilizing hydraulic actuators with sufficiently high bandwidth to generate almost entirely computer control-defined dynamic behaviors. Hydraulics have the benefit of high force generation and very fast response times with a small and lightweight cylinder, allowing the cylinders to be mounted directly on lightweight, strong robot legs. However, there are several drawbacks: first, any motion takes the same amount of power regardless of the load placed on the actuator, because cylinder displacement results in fluid displacement from the high pressure side (pumped) of the system to the low pressure (collected) side of the system. This effect can lead to very energetically wasteful motions if the actuators are moving without significant loading, such as during leg swing retraction. Second, a hydraulic pump, hydraulic lines, and cylinders add significant complexity throughout a machine such as a legged robot. While excellence in engineering makes it possible, fluid leaks and regular maintenance are common in existing state of the art systems.
For existing state of the art hydraulic systems, leg springs are not used and thus gait energy is not stored during stance and liftoff. High-bandwidth and high-power hydraulic actuation allows the dynamic behavior to be controlled, but at a cost: all negative power done by the leg is dissipated as heat, and all positive power must be generated by the actuators. The benefit of such a design choice is greater flexibility in gait development due to nearly direct computer control of leg dynamics, but the price is vastly wasteful energy use. Thus, while the specialized hydraulic actuators are capable of this unnecessary challenge, there is an unavoidable cost of very low efficiency. Hence there remains a need in the art for improved devices and methods for legged robotic locomotion.